In the field of the graphic arts, achievement or satisfactory reproduction of a continuous tone image with dots or a line image requires an image formation system capable of producing a developed image exhibiting superhigh contrast characteristics. A photographic system having a gamma of not less than 10 is particularly desired.
For this purpose, a special developer called a lith developer has been conventionally employed. A lith developer contains hydroquinone as a sole developing agent and, as a preservative, a sulfite in the form of an adduct with formaldehyde so as to have the free sulfite ion concentration controlled at an extremely low level, usually not more than 0.1 mol/liter, in order not to inhibit infectious development. Therefore, the lith developer is extremely susceptible to air oxidation and does not withstand preservation for more than 3 days.
Use of hydrazine derivatives has been proposed for obtaining a high contrast image with a stable developing solution as disclosed in U.S. Pat. Nos. 4,224,401, 4,168,977, 4,166,742, 4,311,781, 4,272,606, 4,211,857, 4,243,739, etc. According to this technique, photographic characteristics of superhigh contrast and high sensitivity can be assured, and the stability of the developer to air oxidation is markedly improved over lith developers since addition of a sulfite to the developer at a high concentration is permissible.
However, although the above-described image formation system is suitable for obtaining high contrast photographic images employing photographic materials having very high sensitivity, this high sensitivity makes it difficult to adopt these systems for development in a bright room. Development in a bright room is normally carried out with photographic materials of low sensitivity which, for reasons discussed below, do not give high contrast images.
With respect to light-sensitive materials having low sensitivity sufficient for development in a bright room, Japanese Patent Application (OPI) Nos. 83038/85 and 162246/85 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") disclose a contrast system using a hydrazine derivative in which a silver halide light-sensitive material contains a water-soluble rhodium salt. However, addition of a rhodium salt in an amount enough to lower sensitivity results in hindrance of the effect of the hydrazine derivative to increase contrast, thus failing to obtain an image of a desired contrast. Further, it is disclosed in Japanese Patent Application (OPI) No. 62245/81 that a high contrast image can be obtained by developing in the presence of a tetrazolium compound to thereby inhibit development in the toe portion of a characteristic curve. This technique, however, involves problems such that the silver halide light-sensitive material containing the tetrazolium compound is easily deteriorated during preservation and is then capable only of providing a low contrast image, and further that a reaction product of the tetrazolium compound formed upon development partly remains in the film, which causes stains and uneven development.
On the other hand, Japanese Patent Application (OPI) No. 157633/84 proposes a process for producing a silver halide photographic emulsion containing a water-soluble rhodium salt in an amount of from 10.sup.-8 to 10.sup.-5 mol per mol of silver halide and an organic desensitizer whose polarogram shows that the sum of the anode potential and the cathode potential is positive. This process succeeds in reducing sensitivity but fails to provide an image having a high contrast sufficient for use in the field to which the present invention is directed. Besides, there is found no suggestion about use of a hyrazine compound in the above-cited patent application.
Use of an organic desensitizer in high contrast silver halide light-sensitive materials containing a hydrazine compound has conventionally encountered great technical difficulty in application, probably for the following reasons. A hydrazine compound is a motive force for bringing a high contrast, taking part in development processing and inducing nucleation infectious development due to its electron donating property. On the other hand, an organic desensitizer is a photoelectron acceptor which accepts a photoelectron upon imagewise exposure to light to hinder formation of a latent image, resulting in reduction of sensitivity. At the same time, since the organic desensitizer also accepts an electron donated by an electron donor, such as a hydrazine compound, during development processing, the nucleation infectious development is inhibited, resulting in the failure of obtaining a high contrast image.